Hardware and Operating System Support for Persistent Memory On A Memory Bus

ABSTRACT

Implementations of a file system that is supported by a non-volatile memory that is directly connected to a memory bus, and placed side by side with a dynamic random access memory (DRAM), are described.

RELATED APPLICATION

This application claims priority to, and is a continuation of, the U.S.Non-Provisional application Ser. No. 12/503,480 to Condit et al.,entitled “Hardware and Operating System Support for Persistent Memory OnA Memory Bus,” filed Jul. 15, 2009, which claims priority to U.S.Provisional Application No. 61/108,400 to Condit et al., entitled,“Cache Hierarchy, File System, and Operating System forByte-Addressable, Non-Volatile Memory,” filed Oct. 24, 2008.

BACKGROUND

A traditional computing device directly connects a volatile memorycomponent, such as dynamic random access memory (DRAM), to a memory bus;however, persistent storage devices, including disk and flash, areconnected to a slow, low bandwidth input/output (I/O) bus. To achieveacceptable performance, a file system aggressively buffers data in theDRAM at the risk of data loss or file system corruption in the event ofa failure (e.g., system crash or power loss). The file system, which maybe a part of an operating system, includes responsibility for managingthe data on the persistent storage.

In an implementation, the file system in the computing device mayprovide consistency guarantees, temporal safety guarantees, correctness,and performance for data and data accesses stored on a device. Theconsistency implemented by the file system assures that data stored inthe persistent storage has not been corrupted, so that the data maydescribe a valid file system. The temporal safety may limit an amount oftime that the data—once written by an application—resides in thevolatile memory before being transferred to the non-volatile memory. Inother words, the temporal safety (guarantee) defines the time between awrite issued by an application, and the write becoming persistent. Theapplication may include a program designed to perform a specific tasksuch as reading or writing data. The correctness describes whetherwrites are reflected to the persistent storage after the writes wereissued by the application. The consistency, the correctness, and thetemporal safety may be sacrificed to a certain degree, in order toovercome performance limitations of the persistent storage.

SUMMARY

The following presents a simplified summary in order to provide a basicunderstanding of some aspects of the disclosed subject matter. Thissummary is not an extensive overview of the disclosed subject matter,and is not intended to identify key/critical elements or to delineatethe scope of such subject matter. A purpose of the summary is to presentsome concepts in a simplified form as a prelude to the more detaileddescription that is presented later.

In an implementation, a computing device may include a non-volatilememory (to provide a persistent storage) that is directly connected to amemory bus (i.e., directly addressable), and placed side by side with avolatile memory. To this end, a file system supported by such a hardwaresetup may be implemented to improve correctness, temporal safety,consistency, and performance in the computing device, through the use ofatomic updates and maintaining the order of writes. The file system mayimplement a tree structure, which allows large amounts of data to beatomically changed.

To accomplish the foregoing and other related ends, certain illustrativeaspects are described herein in connection with the followingdescription and the annexed drawings. These aspects are indicative ofvarious ways in which the disclosed subject matter may be practiced, allof which are intended to be within the scope of the disclosed subjectmatter. Other advantages and novel features may become apparent from thefollowing detailed description when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Thesame numbers are used throughout the drawings to reference like featuresand components.

FIG. 1 is a block diagram of an exemplary computing device architecture.

FIG. 2 is a block diagram of an exemplary control processing unit (CPU).

FIG. 3 is a block diagram of an exemplary cache controller.

FIG. 4 is a block diagram of an exemplary memory controller.

FIG. 5 is a block diagram of an exemplary non-volatile memory filesystem structure.

FIG. 6 is a block diagram illustrating an example of in-place append.

FIG. 7 is a block diagram illustrating an example of atomically updatinga pointer to a copied and modified data block.

FIG. 8 is a block diagram illustrating an example of a partial copy-onwrite.

FIG. 9 is a block diagram illustrating an example of a partialcopy-on-write involving multiple data blocks.

FIG. 10 is a flow diagram of a method for atomically updating a datastructure.

DETAILED DESCRIPTION

This disclosure is directed towards systems, components, techniques andmethods for implementing a byte-addressable, bit alterable persistentstorage (i.e., provided by non-volatile memory) that is directlyconnected to a memory bus, and placed side by side with a dynamic randomaccess memory (DRAM) component. In an implementation, a phase changememory (PCM) may be the byte-addressable, bit alterable persistentstorage that is directly connected to the memory bus. In order toaddress the issues of consistency, safety, correctness, and performance,a file system (which is a part of an operating system) may manage dataon the non-volatile memory. The file system may be used to organize thedata in the non-volatile memory while improving consistency, safety,correctness, and performance. The improvement may be implemented by thefile system through “ordering” and “atomicity” provided in the hardwaresetup. For example, the “ordering” provided by the hardware setup allowssoftware to declare important ordering constraints between writes tonon-volatile memory, ensuring that any writes reordered by a cachecontroller or a memory controller do not violate these constraints. The“atomicity” may assure correctness and the safety by writing data of aparticular size (e.g., 8 bytes) either completely to the non-volatilememory or not at all.

A file system may be utilized with the non-volatile memory component tofacilitate consistency and safety guarantees, as well as improvedperformance relative to disk-based file systems running on the samestorage medium. For example, the file system can facilitate reading andwriting of its data structures directly from the non-volatile memorycomponent, as opposed to conventional file systems that maintain copiesof data structures in volatile memory and then reflect changes to thosedata structures to non-volatile memory.

Furthermore, an operating system (OS) may utilize the non-volatilememory component to facilitate performing an “instant boot” (of theoperating system). In an implementation, the instant boot uses thenon-volatile memory component to provide persistent storage for OS datastate. For instance, a snapshot (e.g., memory image) of the DRAM can betaken after the computer has booted and can be stored in thenon-volatile memory component. On the next boot, the snapshot can becopied from the non-volatile memory component to the DRAM to facilitatebypassing at least a portion of the boot sequence.

In accordance with various other aspects and embodiments, thenon-volatile memory component may be employed to facilitate efficientlyupdating a file system tree when a block (e.g., data block) in the filesystem tree is updated; to facilitate employing a fast-append operationto take advantage of any available unused space in a last data block ofa file, where an append operation that can fit within an unused portionof a data block can be written directly to that part of the data block,and the file size can be updated atomically in order to commit thechange; and/or to facilitate employing a fast copy operation, where acopy-on-write operation can be improved by copying a desired portion ofthe data (e.g., only copying the data that is to be preserved). Inanother implementation, the non-volatile memory component is utilizedwith regard to a set of uses for applications. For example, thenon-volatile memory component is utilized to facilitate checkpointingapplication state, where an application can reflect the state of theapplication (e.g., heap, stack, registers) directly to the non-volatilememory component. The application can utilize such checkpoints tofacilitate reliability and security.

Computing Device Architecture

FIG. 1 is a computing device architecture 100 that includes centralprocessing unit or CPU component 102, cache controller component 104,northbridge component 106, memory controller component 108, non-volatilememory component 110, dynamic random access memory (DRAM) component 112,southbridge component 114, and a hard disk component 116. The CPUcomponent 102 is a control unit in the computing device 100. In animplementation, the CPU component 102 includes the cache controllercomponent 104, and the northbridge component 106 includes the memorycontroller component 108. In other implementations, the CPU component102 may be an integrated system that includes the cache controllercomponent 104 and the memory controller 108; such an integrated systemwould connect the non-volatile memory component 110, DRAM component 112,and southbridge component 114 directly to CPU component 102. Duringapplication writes (i.e., writes) for which the writes may includespecific tasks, such as writing data to implement or execute a program,the computing device 100 implements a file system to assure temporalsafety, correctness, consistency, and performance. The file system maymanage data in the computing device 100.

The CPU component 102 may include a system that integrates the cachecontroller component 104 and the memory controller 108. The file systemmay be implemented by enforcing “ordering” and “atomicity” in a memorysystem of the computing device 100. The memory system may include theCPU component 102 that may directly access the non-volatile memorycomponent 110. In an implementation, data from the non-volatile memorycomponent 110 may be read into the CPU component 102 without beingreflected in the DRAM component 112.

In an implementation, the “ordering” includes a sequence of writes (in acertain order) that are issued by the file system. The computing device100 stores the sequence of writes temporarily in the cache controllercomponent 104, and then flushes the sequence of writes from the cachecontroller component 104 into the memory controller component 108,possibly in a different order to improve performance. In otherimplementations, the cache controller component 104 and the memorycontroller component 108 may preserve ordering for certain data writes,ensuring that the data writes are received by the non-volatile memorycomponent 110 in the same order that the data writes were issued by thefile system. The file system uses the “ordering” guarantee to enforcesafety, consistency, and correctness guarantees. Furthermore, the“ordering” may include modification of the CPU component 102 e.g., usingepoch identification (epoch ID), persistence bits, etc. as furtherdiscussed below.

The “atomicity” may include atomic write to non-volatile memorycomponent 110 that may either be completed entirely, or not at all. Inan implementation, the cache controller component 104 identifies atomicwrites and send this information to the memory controller component 108,which enforces atomicity for these data writes. In an implementation,the file system uses the atomicity to help enforce safety, consistency,and correctness guarantees.

The CPU component 102 includes a device or component (not shown) thatinterprets and executes instructions (such as writes) issued by anapplication. Epochs indicate ordering constraints on writes;specifically, the hardware must preserve ordering between two writesthat occur in different epochs (i.e., writes in the same epoch may bereordered arbitrarily). The CPU component 102 includes a processor corethat may provide the epoch ID. The epoch ID is used as a reference tomaintain the “ordering” in the cache controller component 104 and thememory controller component 108. The epoch ID indicates a current epoch,for which the CPU component 102 is writing to the non-volatile memorycomponent 110. The current epoch includes all writes issued to thenon-volatile memory component 110 since the previous epoch boundary, asindicated by the application. In an implementation, the CPU component102 issues writes to a cache (not shown) in the cache controllercomponent 104 (e.g., CPU component 102 writes “A” to the cache in thecache controller component 104 where “A” is a variable). In turn, thecache controller component 104 stores the data to cache blocksidentified by the epoch ID. Subsequently, the cache controller component104 may perform writes to the memory controller component 108 (e.g.,cache controller component 104 writes “A” to memory controller component108). The transfer and processing of the data from the cache controllercomponent 104 to the memory controller component 108 may be performedaccording to the ordering indicated by the epoch. The memory controllercomponent 108 may write the data to the non-volatile memory component110, or to the DRAM component 112. The writes performed by the cachecontroller component 104 and the memory controller component 108 maymaintain “ordering” according to the epoch boundaries given by theapplication running in the computing device 100.

The cache controller component 104 may include a memory subsystem inwhich frequently used data values may be duplicated for quick access.The cache controller component 104 includes the cache block that storescontent of frequently accessed random access memory (RAM) locations, andaddresses where the RAM locations may be stored. The cache controllercomponent 104 also stores the content of non-volatile memory locations.When the CPU component 102 references an address in the non-volatilememory component 110, the cache controller component 104 may verify ifthe address is held in the cache block. If the address is held in thecache block, then the contents at that address (i.e., data) may bereturned to the CPU component 102.

The cache controller component 104 implements mechanisms to enforce the“ordering” and the “atomicity” in the computing device 100. As furtherdiscussed below, the cache controller component 104 may use persistencebits, epoch IDs, and atomic bits in order to enforce “ordering” withinthe cache controller component 104.

The northbridge component 106 may include an interface between the CPUcomponent 102, the nonvolatile memory component 110, and the DRAMcomponent 112. In an implementation, the northbridge component 106 is asystem that includes the memory controller component 108. The memorycontroller component 108 may include flexibility of access to thevolatile memory (e.g., DRAM component 112) and the non-volatile memorycomponent 110. Data may be accessed at the same time on both components(i.e., non-volatile memory component 110 and DRAM component 112). Inaddition, the memory controller component 108 may be responsible fordifferentiating non-volatile component 110 (e.g., PCM) operations fromDRAM component 112 operations by handling distinct timingcharacteristics of both technologies (i.e., non-volatile memorycomponent 110, and DRAM component 112). These properties of the memorycontroller component 108 remain true in an alternative implementationwhere the memory controller component 108 is integrated into the CPUcomponent 102, and the northbridge component 106 is not present.

In the same manner that the cache controller component 104 implementsthe mechanism to enforce “ordering” and “atomicity,” the memorycontroller component 108 implements the “ordering” and the “atomicity”during operation. The “ordering” in the memory controller component 108is implemented by the use of the epoch ID, the persistence bits, and atiming provided by a scheduler component (as discussed below). In animplementation, the scheduler component performs scheduling of dataaccesses, or provides the timing in data access within the computingdevice 100.

In another implementation, for correct operation (using “atomicity”),the memory controller component 108 provides support for atomic writes(i.e., “atomicity”) in the file system (or other software components).For example, in case of a power outage or failure, in-flight writes maybe either completed in their entirety or fail completely. The in-flightwrites may include persistent data in the cache blocks of the cachecontroller component 104 that were not yet reflected in the non-volatilememory component 110. In an implementation, the atomic writes in thefile system are obtained by inserting a capacitor device(s) (not shown)in the memory controller component 108, to assure that there is enoughenergy to complete a maximum number of write transactions ongoing withina subsystem of the non-volatile memory component 110. As a result,although the memory controller component 108 may fail to issue furthercommands, in-flight writes within the non-volatile memory component 110may be guaranteed to complete.

The non-volatile memory component 110 may include a reliable storagesystem (i.e., handles more memory requests) that does not lose data whenpower is removed. The non-volatile memory component 110 may be abyte-addressable and bit alterable non-volatile memory directlyconnected to a memory bus (i.e., memory bus 118) to obtain direct accessor exposure to the CPU component 102. The byte-addressability mayinclude the ability of the non-volatile memory component 110 to performsmall, random writes as compared to large data transfers in traditionaldisk or flash. In contrast to non-volatile flash memories operating oncoarse blocks (e.g., kilobytes of data), the byte addressabilityfunction may improve performance and reduce power costs. The bitalterable function may include data writing in the non-volatile memorycomponent 110 without separate data erasures. In an implementation, aphase change memory (PCM) may be implemented as the non-volatile memorycomponent 110. The non-volatile memory component 110 may support thefile system that optimizes the properties of the byte-addressable andbit alterable non-volatile memory component 110. In addition, the filesystem may exploit small, random writes at a given time in thenon-volatile memory component 110. This file system may optimize the useof the small, random writes whenever possible to reduce memory bustraffic and unnecessary writes in the non-volatile memory component 110.

In an implementation, the non-volatile memory component 110 is a type ofnon-volatile memory that provides non-volatile, byte-addressable storage(i.e., persistent storage). Unlike the DRAM component 112, thenon-volatile memory component 110 may store data by using resistivity asopposed to electrical charge. The non-volatile memory component 110 mayalso use some other physical property that allows it to expose byteaddressable, bit-alterable persistent storage. In an embodiment,byte-addressable non-volatile memory component 110 simultaneouslyimproves performance and strengthens temporal safety, correctness, andconsistency guarantees. Trade-offs that traditional file systems makebetween these factors (i.e., temporal safety, correctness, etc.) may bebased on the properties of hard disks (e.g., hard disk 116), whichgreatly favor sequential access of large data blocks. With the byteaddressable non-volatile memory component 110, a different set oftrade-offs in the file system may be explored. The different set oftrade-offs in the file system may simultaneously improve the performanceand strengthen temporal safety, correctness, and consistency guaranteesby enforcing the “ordering” and “atomicity” in the CPU component 102.

The non-volatile memory component 110 and the DRAM component 112 may bedirectly connected to a memory bus 118. The memory bus 118 may carrydata to or from the northbridge component 106. The northbridge component106 may further connect to the CPU component 102 through a signal path120. The signal path 120 may carry the data to or from the CPU component102. In other implementations, the northbridge component 106 is notincluded in the computing device 100. To this end, the memory controllercomponent 108 may be integrated to the CPU component 102 with directsignal paths between the NVM component 110 and the DRAM component 112.

The DRAM component 112 may be used for the heap and the stack to furtherprotect lifetime wear of the non-volatile memory component 110. Inaddition, the use of the DRAM component 112 (i.e., for the heap and thestack) may provide power reduction consumption in the computing device100. The heap may include a portion of memory reserved for a program touse for the temporary storage of data structures whose existence or sizecannot be determined until a program/application is running. The stackmay store data such as procedure and function call addresses, passedparameters, and sometimes local variables.

The southbridge component 114 may include an interface that connects thenorthbridge component 106 and I/O devices such as hard disk component116. The southbridge component 114 may pass data to or from thenorthbridge component 106 through a signal path 122. At the other sideof the southbridge component 114 is the hard disk component 116.Although the hard disk component 116 is shown, other implementations mayuse different devices or no devices at all. The hard disk component 116may be a non-volatile storage device that stores digitally encoded data.In an implementation, the hard disk component 116 passes or receivesdata through a signal path 124 that is connected to the southbridgecomponent 114. This data may subsequently be accessed through the signalpath 122 by the northbridge component 106.

Central Processing Unit

FIG. 2 is an exemplary implementation of a central processing unit (CPU)component 102 (FIG. 1) that includes processor cores 200-1, 200-2, . . ., 200-n (hereinafter referred to as processor 200), and epoch IDcounters 202-1, 202-2, . . . 202-n (hereinafter referred to as epoch IDcounter 202). The processor 200 may include an electronic circuit thatcan execute write operations (writes). To maintain ordering amongwrites, each processor cores 200-1, 200-2, . . . 200-n, may use an epochID counter 202 that provides an epoch ID used as a reference to enforceordering in the non-volatile memory component 110. The epoch ID mayindicate the writes that the processor 200 may be writing to thenon-volatile memory component 110. The epoch ID may be supplied to thecache controller component 104, and the memory controller component 108,in order to detect and prevent ordering violations. As discussed furtherbelow, the epoch ID may be used by the file system to enforce theordering that is supported by the computing device 100.

In an implementation, the epoch ID counter 202 may be incremented by one(1) each time the processor cores 200-1, 200-2, . . . 200-n encounters amemory barrier that marks the end of the epoch ID. To this end, theepoch ID may allow the file system (as further discussed below) todetect a write that may be safely reordered (because of the memorybarrier). When the write commits to any address in the cache controllercomponent 104, the write may be tagged with the value of the epoch IDprovided by the epoch ID counter 202. The value of the epoch ID may bepropagated with the write throughout the cache controller component 104and memory controller component 108 to enforce the ordering.

Cache Controller Component Ordering

FIG. 3 is an exemplary implementation of a cache controller component104 (FIG. 1). Caching persistent data may increase performance becausethe non-volatile memory component 110 writes slower than the cachecontroller component 104. In addition, since non-volatile memorycomponent 110 cells may sustain a limited number of writes beforewearing out, reducing write traffic through caching of the persistentdata may extend the lifetime of the non-volatile memory component 110.The cache controller component 104 may include cache blocks 300-1,300-2, . . . and 300-n (hereinafter referred to as cache 300) thatstores data.

Each cache blocks 300-1, 300-2, . . . 300-n may be associatedrespectively with persistence bits 302-1, 302-2, . . . 302-n(hereinafter referred to as persistence bit 302). The persistence bit302 (or vector bit) may be set appropriately at the time the cache 300is filled based on cache's address. The persistence bit 302 may be usedto identify the cache 300 data referenced to the non-volatile memorycomponent 110 address ranges. In other words, if the cache 300 (e.g.,cache block 300-1) includes the data to be written into the non-volatilememory component 110, then the persistence bit 302 may be required toidentify the cache 300 that contains the data (i.e., persistence bit isequal to one). Otherwise, the persistence bit 302 may not be required tobe associated with the cache 300 (i.e., persistence bit is equal tozero), if the data may be referenced to the DRAM component 112.

Epoch IDs 304-1, 304-2, . . . 304-n (hereinafter referred to as epoch ID304) may refer to a defined memory barrier (for each epoch ID 304) asprovided by the epoch ID counter 202 in the CPU component 104. In animplementation, the epoch ID 304 is used to identify the cache 300 whenthe persistence bit 302 is set to one. In other words, the data in theepoch ID 304 may be referenced to the non-volatile memory component 110.For a given epoch ID 304 (e.g., epoch ID 304-1), the epoch ID 304 mayrefer to one or more writes of persistent data before a defined memorybarrier that were not yet reflected to the non-volatile memory component110. These writes of persistent data may be referred to, collectively,as belonging to an in-flight epoch identified by epoch ID 304. The oneor more dirty persistent data in the in-flight epoch ID 304 may betracked by a dirty block counter 306. The dirty counter 306 may includedirty counters 306-1, 306-2, . . . 306-n (hereinafter referred to asdirty block counter 306) that may be associated respectively with thein-flight epoch ID 304 to implement the ordering in the cache controllercomponent 104.

In another implementation, the dirty block counter 306 may track thenumber of persistent dirty data residing in the cache 300 for eachin-flight epoch ID 304 at each point in time. In a first in first out(FIFO) buffer, the oldest in-flight epoch ID 304 may include the datathat were written or entered earlier. The cache controller component 104tracks which epoch ID is assigned to the oldest in-flight epoch. Thedirty block counter 306 may be incremented, whenever the data is updated(i.e., a new write application provides a new data), and the dirty blockcounter 306 may be decremented when the dirty persistent data isreflected to the non-volatile memory component 110. When the dirty blockcounter 306 associated with the oldest in-flight epoch ID 304 reaches azero value, then the in-flight epoch ID previously identifying theoldest epoch ID no longer resides in cache controller component 104; thecache controller component 104 then identifies the next oldest epoch ID.

In another embodiment, the cache controller component 104 performs anupdate to the oldest in-flight epoch ID 304 in the cache 300. At eachpoint in time, any of the in-flight epoch ID 304 that are younger thanthe oldest in-flight epoch ID 304 in the cache 300 may not be replaced.To this end, a replacement is performed in a manner that respectsordering in the cache 300.

Addresses mapped to the DRAM component 112 and persistent blocks fromthe oldest epoch in the cache 300 may all be eligible for replacement.If the cache controller component 104 does not find data blocks mappedto the DRAM component 112 for replacement, then the cache controllercomponent 104 may attempt to replace the oldest in-flights epoch ID 304in the cache 300. To this end, all of the earlier in-flight epoch ID 304may be flushed from that level of cache hierarchy first, and in programorder.

Memory Controller Component Ordering

FIG. 4 is an exemplary implementation of a memory controller component108 (FIG. 1). A memory controller 400 may include memory controllertransactions 400-1, 400-2, . . . and 400-n (hereinafter referred to asmemory controller 400) that may contain metadata. The memory controller400, following the cache 300, may be respectively associated withpersistence bit 402 (i.e., persistent bits 402-1, 402-2, . . . 402-n),epoch ID 404 (i.e., epoch IDs 404-1, 404-2, . . . 404-n), dirty blockcounter 406 (i.e., dirty block counters 406-1, 406-2, . . . 406-n), andatomic bits 408 (i.e., atomic bits 408-1, 408-2, . . . 408-n).

In an implementation, the memory controller component 108 may assurethat a write (e.g., dirty data) may not be reflected to the non-volatilememory component 110 (e.g., PCM) before in-flight writes associated withall of the earlier epochs are performed. As such, the memory controllercomponent 108 may record the epoch ID 304 associated with eachpersistent write in the memory controller's transaction queue, andmaintain a count of the in-flight writes from each epoch that is queuedup at a given point in time (i.e., by using the dirty counter block406). The persistence bit 402, the epoch ID 404, and the dirty blockcounter 406 may include the same functions and operations as thepersistence bit 302, the epoch ID 304, and the dirty block counter 306,which were discussed under the cache controller component 104. Inanother implementation, among persistent writes, only those persistentwrites associated with the oldest epoch (e.g., in epoch ID 404) may beeligible for scheduling in the memory controller component 108 at anypoint by a scheduler component 410.

The memory controller 400 may further include scheduler component 410that schedules memory accesses according to timing constraints definedby a particular memory technology. The scheduler component 410 may beused in the memory controller 400 to guarantee correctness by followingthe program order to enforce ordering in the cache controller component104. The scheduler component 410 may further support the enforcement ofthe ordering by the file system (or other software components) throughintegrated access (i.e., at the same time) of the non-volatile memorycomponent 110, and the DRAM component 112. Since the memory controller400 follows a queue of requests from the cache 300, the schedulercomponent 410 assures that the writes cannot be reflected in thenon-volatile memory component 110 before in-flight writes with all ofearlier epochs are performed. The queue of requests from the cache 300may include flushing out data information (i.e., reflecting informationto the memory controller 400 and then deleting this information from thecache 300) received by the memory controller component 108 according tothe timing as designed in the file system. The scheduler component 410may include an added state for tracking of the persistence bit 402 andthe epoch ID 404. The timing, persistence bit 402, and the epochs ID 404may be referred to as restraints (or conditions) used to enforceordering in the file system while scheduling memory accesses.

Atomicity

The atomicity may be enforced at the memory controller component 108 toassure safety and correctness guarantees during enforcement of the“ordering.” To prevent data corruption during unexpected failures, thefile system atomically updates certain memory location in thenon-volatile memory component 110. In an implementation, in case of apower outage or failure, in-flight writes must be either completed intheir entirety or must fail completely, and not update the non-volatilememory component 110. In certain implementations, a capacitor device(not shown) may be inserted in the memory controller component 108. Thecapacitor device may hold enough energy to complete the maximum numberof write transactions ongoing within the non-volatile memory component110.

In another implementation, atomic persistence writes in the memorycontroller component 108 are provided via hardware journaling. In otherwords, a write to a predetermined address may signal to the memorycontroller component 108 that the next write in the program order may beperformed atomically. The hardware journaling may be implemented usingthe atomic bit 408 associated to further identify each memory controllertransactions 400-1, 400-2, . . . 400-n. The atomic bit 408 may provide atiming of the atomic write in the program order in the memory controllercomponent 108. The atomic bit may be propagated throughout the cachecontroller component 106, and may be interpreted appropriately by thememory controller component 108.

Design Principles for a Non-Volatile Memory File System

Using the non-volatile memory component 110 instead of a hard disk 116may provide performance benefits based on the speed of the non-volatilememory component 110. The non-volatile memory component 110 alsoprovides an opportunity to further improve both performance andreliability by designing a non-volatile memory file system (PFS)optimized for the unique properties of the byte-addressable,non-volatile memory component 110.

In an implementation, the PFS may be based on three design principles.The first design principle is to exploit small, random writes. Insteadof writing large blocks of data at a time, PFS is optimized to usesmall, random writes whenever possible, to reduce memory bus traffic andunnecessary writes to the non-volatile memory component 110.

The second design principle is to avoid using the DRAM component 112 forfile system data and metadata. Instead, PFS stores data and metadata inthe non-volatile memory component 110. This frees the DRAM component 112for other uses, and provides the opportunity to reduce power consumptionby reducing the amount of the DRAM component 112 required by thecomputer device architecture 100. Furthermore, the operating system doesnot need to manage two tiers of storage, which simplifies the task ofensuring reliability.

The third design principle is to rely on hardware assistance toimplement guarantees. Since the non-volatile memory component 110provides the opportunity to eliminate the layer of the DRAM component112 between the CPU component 104 and persistent storage, interposing onapplication operations to enforce ordering or safety guarantees couldimpede performance. To this end, the PFS can be designed on the premisethat hardware enforces the ordering and temporal safety guarantees ofdata written into the CPU cache. PFS uses write barriers to denote therequired ordering between sets of operations and to mark certain writesas atomic. The underlying cache controller component 104 and the memorycontroller component 108 are then free to issue writes between two writebarriers in any order while still preserving consistency guarantees.

The above design principles, coupled with the architectural design forthe non-volatile memory component 110, allow for the design of a filesystem that provides strong consistency, correctness, and temporalsafety guarantees. PFS provides a strong consistency guarantee byassuring that a crash or power failure will not result in a corruptedfile system image. PFS also provides a strong correctness guarantee byleveraging architectural support to reflect application writes to thenon-volatile memory component 110 atomically and in the order they wereissued by the application. Finally, PFS may improve temporal safetyguarantees by reducing the window of vulnerability for data loss fromseconds to the number of cycles required make data persistent in PRAM.

A File System Layout for Non-Volatile Memory

In an embodiment, file system data and metadata may be stored in a treestructure in the non-volatile memory component 110, accessible from aroot pointer stored at a predetermined location. Consistency is assuredthroughout the file system by performing intermediate operations inunused portions of the non-volatile memory component 110 and then usingan atomic operation to commit them. For example, when changing a page ofuser data, PFS copies the existing user data to a freshly allocatedblock of the non-volatile memory component 110, updates the new block,and then atomically overwrites the pointer to the old block with thepointer to the new block. In many cases, updates can be done entirelyin-place through careful ordering of writes. In an embodiment, PFS marksepoch boundaries before and after each 64-bit “commit” of file systemstate, which assures that the committing operation will be written tothe non-volatile memory component 110 only after the write operationsupon which the committing operation depends have been made persistent.

As a result, there is a very strong consistency guarantee: all filesystem operations are either committed completely or not at all. Inaddition, there are strong safety guarantees. Since updates can beapplied to the non-volatile memory component 110 synchronously, datawill arrive in the non-volatile memory component 110 in the time ittakes to flush the cache. Since most common file operations can beperformed in-place, high performance gains may be achieved.

In an embodiment, persistent data structures within PFS include at leastthree kinds of files. First, an inode file is a single file containingan array of fixed-size inodes, each uniquely representing a file ordirectory in the file system. The root of the inode file represents theroot of the file system as a whole, and this root pointer is stored in apredetermined location in the non-volatile memory component 110. Inodescontain file metadata including the root pointer and size of theassociated file. An entry in the inode file is only considered valid ifit is referred to by a valid directory entry. Second, directory filescontain an array of directory entries that include an inumber (i.e., theindex of an inode in the inode file) and the name of the correspondingfile. Directory entries are only considered valid if they contain anon-zero inumber. Third, data files contain user data only.

FIG. 5 is an exemplary block diagram 500 of the non-volatile memorycomponent 110 (FIG. 1) file system structure. The top half of the filesystem is the inode file 502, which starts from a root pointer 504 at apredetermined location. The dashed box shows the “data” for the inodefile 502, which includes an array of inodes 506. Each inode points to adirectory file or a data file. FIG. 5 shows three such files, whose datais also stored in a hierarchal structure.

In an embodiment, each kind of file is represented with the same basicdata structure: a tree consisting entirely of, e.g., 4 KB blocks. Theleaves of the tree represent a file's data (i.e., user data, directoryentries, or inodes), and the interior nodes of each tree contain 51264-bit pointers to the next level of the tree. In FIG. 5, the leaves ofeach file are shown in a dashed box; taken in sequence, the blocks inthis dashed box represent the file's contents. For simplicity, only twopointers per block are shown.

The height of each tree is indicated by the low-order bits of the rootpointer 504, which allows the file system to determine whether a givenblock is an interior (pointer) block or a leaf (data) block byremembering the number of hops taken from the root pointer 504. Forexample, with a tree of height of zero, the root pointer 504 pointsdirectly to a data block which can contain up to, e.g., 4 KB of filedata. With a tree of height one (1), the root pointer 504 points to aninterior block of 512 pointers, each of which points to a 4 KB datablock, for a total of, e.g., 2 MB. A tree of height three (3) can store1 GB of data, and a tree of height 5 can store, e.g., 256 TB of data. Itis to be noted that a given tree is of uniform height. For example, if atree has height of three (3), then all file data will be found threehops down from the root pointer 504, and no file data is stored atinterior nodes. It is also noted that because the root pointer 504 andits height are stored in one 64-bit value, they can be updatedatomically.

At any level of the tree, a null pointer represents zero data for theentire range of the file spanned by that pointer. For example, if theroot pointer 504 is a null pointer with height five (5), then itrepresents an empty (i.e., zeroed) 256 TB file. Null pointers can alsoappear at interior nodes, so a write to the end of this 256 TB file willnot cause us to write 256 TB of zeros; rather, it will result in a chainof five pointers down to a single data block, with null pointers in theremainder of the interior nodes. Thus, the file representation canachieve very compact representations of large, sparse files.

Trees can have varying height. For example, data file 1 508 has heightone (1), directory file 2 510 has height 2, and data file 3 512 hasheight three (3). Data blocks may be at the same level of each tree. Forexample, in directory file 2 510, the third data block is still locatedthree hops from the root, even though the parent of the third data blockonly has one pointer. It is noted that data file 3 512 is missing asecond block due to a null pointer in the parent—this block is assumedto be entirely zero. Components of the tree can store pointers 514, filemetadata 516, directory information 518, or user data 520.

The size of each file is stored along with each root pointer. For theinode file 502, the file size is stored in a predetermined location(root node 504). For all other files, the file size is stored in eachfile's inode next to the root pointer 504. If the file size exceeds theamount of data encompassed by the current tree height, then the tail ofthe file is assumed to be zero. Therefore, the 256 TB zero file may alsobe represented by a tree of height of zero (0) and a file size of 256TB. If the file size is less than the amount of data represented by thecurrent tree height, then data in the tree beyond the end of the file isignored and may contain garbage. For example, if a tree has a height 1(with a maximum of 2 MB) and a file size of 1 MB, then the first 256pointers of the interior node point to valid data, and the last 256pointers are ignored and may contain arbitrary bits.

In an implementation, persistent data can be updated in three ways:in-place updates, in-place appends, and partial copy-on-write. Theseapproaches to updating data represent distinct advantages overdisk-based file systems, which are not able to modify persistent storageat a byte granularity.

Updating Persistent Data Via In-Place Update

In-place updates are an efficient approach. In an embodiment, for datafiles, in-place updates can be performed for writes of 64 bits or less,since the hardware guarantees that these updates are atomic. The filesystem can be built around any granularity of atomic writes. For thepurposes of discussion, we will assume an atomic write size of 64 bits.For metadata structures, file system invariants can be used to doin-place updates. For example, when adding an entry in a directory file,the file system can find an unoccupied (i.e., zeroed) directory entry,write the name of the new entry, and then write the entry's number.Since an entry is only considered valid when it contains a non-zeroinumber, this final write commits the change to the file system.Similarly, inodes are not considered valid until a directory entrypoints to them, so inodes that are not yet live can be modifiedin-place. For example, the file system may be writing to a “dead” inodein preparation for creating a directory entry that points to the “dead”inode.

FIG. 6 is an exemplary implementation 600 of an in-place update. In oneembodiment, the non-volatile memory component 110 (FIG. 1) contains filedata stored in a tree structure, with a root 602 at the beginning of thetree. The root 602 has a root pointer 604 that points to pointer block 1606. Pointer block 1 606 contains multiple pointers, including pointer608, which points to pointer block 2 610. Pointer block 2 610 haspointer 1 612 which points to data block 1 614 and pointer 2 616 whichpoints to data block 2 618. Other branches of the tree may containadditional pointers and data blocks. For example, pointer block 3 620,data block 3 622, and data block 4 624 are in another branch. The datafor File 1 626 consists of the data stored in data blocks 622, 624, 614,and 618. The size of modified data 628 is less than or equal to themaximum size that can be atomically written. A particular amount of datacan be atomically written if there is a guarantee that the entire writeoperation will either succeed or fail (i.e. 8 bits, 64 bits, 8 bytes).Since modified data 628 is changed through an atomic write, no othermodifications to the tree structure are needed when the changes are madeto Modified Data 628.

Updating Persistent Data via In-Place Appends

In-place appends take advantage of the file size variable thataccompanies the root pointer for each file. Since all data beyond thefile size is ignored, the file system can safely write to theselocations in-place, and once all of the data has been written, the filesystem can atomically update the file size to extend the valid datarange.

FIG. 7 is an exemplary implementation 700 of an in-place append to afile with root pointer 602. In one embodiment, File 1 626 consists ofdata blocks 622, 624, 614, and 618. Pointer block 2 610 has pointer 2616 which points to data block 2 618 and pointer 1 612 points to datablock 1 514. Appended data 702 is added to data block 2 618, which ispart of File 1 626. A file size variable accompanies the file to theroot pointer 602. Once all of the appended data 702 is written to datablock 2 618, the file size variable is atomically updated to a largerfile size. This extends the valid data range. Any data beyond the filesize is ignored. Therefore, if a crash occurs before the file size isupdated or while the append is in progress, the appended data is simplyignored.

Updating Persistent Data Via Partial Copy-on-Write

Partial copy-on-write is a technique for updating persistent data,allowing an atomic update to an arbitrarily large portion of the filesystem. In this approach, the file system performs a copy-on-write onall portions of the tree that will be affected by the write operation,up to the lowest point at which a change can be committed with a singlewrite.

FIG. 8 is an exemplary implementation 800 of partial copy-on write to afile with root pointer 602. File 1 626 consists of data blocks 622, 624,614, and 618. Pointer block 2 610 includes pointer 2 616, which pointsto data block 2 618. When data block 2 618 needs to be modified, datablock 3 802 is created by copying data block 2 618 to a newly-allocatedlocation in non-volatile memory. In certain embodiments, only some ofthe data in data block 2 618 is copied (i.e., data that will not bemodified). As shown, data block 3 802 is changed in two areas: modifieddata 1 804 and modified data 2 806. After the data in data block 3 802is updated, pointer 2 616 is atomically updated to pointer 3 808, suchthat pointer block 2 610 points to data block 3 802 instead of datablock 2 618. If a crash occurs before pointer 2 616 is updated, pointerblock 2 610 will continue to point to data block 2 618. Therefore,pointer 2 616 will not point to “dirty data” if a crash occurs.

FIG. 9 is an exemplary implementation 900 of a partial copy-on-writeinvolving multiple data blocks. File 1 626 consists of data blocks 622,624, 614, and 618. Pointer block 1 606 includes pointer 608, whichpoints to pointer block 2 610. Pointer block 2 610 includes pointer 1612, which points to data block 1 614 and pointer 2 616, which points todata block 2 618. When data is modified in both data block 1 614 anddata block 2 618, a Subtree 902 is created by copying pointer block 2610 to pointer block 3 904, copying data block 1 614 to data block 3906, and copying data block 2 618 to data block 4 908, where data blocks904, 906, and 908 are newly-allocated blocks in non-volatile memory. Inthis example, a user wants to write data that spans both the third andfourth data blocks of the file. To do so, the file system allocates newspace in the non-volatile memory component 110 for new blocks, copiesany existing data that will not be overwritten (e.g., the beginning ofthe third data block), and then updates the new blocks as appropriate.The file system also copies and updates any pointer blocks that cannotbe updated atomically.

Pointer 3 912 points to data block 3 906 and pointer 4 914 points todata block 4 908. Once the file system has created these data blocks andcopied any data that will not be modified, it writes modified data 916to the new blocks. In addition, it writes modified pointers 912 and 914to block 904, so that block 904 now points to blocks 906 and 908 insteadof 614 and 618. There may still be pointers in pointer block 904 thatpoint to old data blocks (not shown in this example), as long as thosedata blocks have not been modified. After modified data 916 has beenupdated in the Subtree 902, pointer A 608 is atomically updated topointer B 918, so that pointer block 1 606 points to pointer block 3 904instead of pointer block 2 610. If a crash occurs before pointer A 608is updated, pointer A 608 will continue to point to pointer block 2 610.Only when all of the updates in Subtree 902 are complete does the filesystem commit the change by performing an atomic update of the pointer A608. Therefore, pointer block 1 606 will not point to “dirty data” if acrash occurs.

Updating Persistent Data

FIG. 10 is a flow diagram of an exemplary process 1000 for atomicallyupdating a data structure. Process 1000 may be implemented by CPUcomponent 102 of computing device architecture 100. Process 1000 isillustrated as a collection of blocks in a logical flow diagram, whichrepresents a sequence of operations that can be implemented in hardware,software, or a combination thereof. In the context of software, theblocks represent computer instructions that, when executed by one ormore processors, perform the recited operations.

The order in which the method is described is not intended to beconstrued as a limitation, and any number of the described method blockscan be combined in any order to implement the method, or alternatemethod. Additionally, individual blocks may be deleted from the methodwithout departing from the spirit and scope of the subject matterdescribed herein. Furthermore, the process can be implemented in anysuitable hardware, software, firmware, or a combination thereof, withoutdeparting from the scope of the invention.

At step 1002, an application issues a write command. In one embodiment,e.g., 8 bytes of data can be written atomically. Other embodiments mayatomically write smaller or larger amounts of data. At step 1004, if theamount of data to be written is less than or equal to 8 bytes, then anatomic write is issued at step 1006. The method 1000 then ends at step1008.

At step 1002, if the amount of data to be written is more than 8 bytes,then at step 1010 the file system determines if the write is limited toone data block. If so, then at step 1012 the file system determines ifthe write is limited to appending data to a file. If so, then at step1014 the files system appends data to the data block. At step 1016,after the data is appended to the data block, the file system atomicallyupdates a file size variable associated with the appended file. Themethod 1000 then ends at step 1008.

At step 1012, if the write is not limited to simply appending data, thenat step 1018, a copy of the data block is created. In some embodiments,only data that will remain unchanged is copied into the newly createddata block. At step 1020, the file system writes to the data block. Atstep 1022, after all writes to the data block are complete, the filesystem atomically updates the pointer from the original data block tothe new data block. The method 1000 then ends at step 1008.

At step 1010, if the write is not limited to one data block, then atstep 1024 the file system creates a copy of the subtree that containsdata to be modified. At step 1026, the file system writes data to thedata blocks. At step 1028, after all writes to the data blocks arecomplete, the file system atomically updates the pointer from theoriginal subtree to the new subtree. The method 1000 then ends at step1008.

Using Non-Volatile Memory with Volatile Data Structures

In an embodiment, for speed and convenience a number of data structuresmay be maintained in volatile memory. First, the file system may have alist of free blocks of the non-volatile memory component 110 as well asfreed and allocated inumbers. Because these data structures are storedin volatile memory (i.e., the DRAM component 112), they arereconstructed from the file system at every boot; however, this can bedone in a fraction of a second, even on a moderately full file system.Storing this data in the DRAM component 112 provides that there is noneed to worry about consistency guarantees between the free list and thefile system itself.

Second, the file system stores a list of freed and allocated blocks froman in-flight copy-on-write operation. For example, while performing awrite, the file system will keep track of any newly allocated blocks(i.e., FIG. 9, block 904, block 906, block 908) as well as any blocksthat will need to be freed if the operation succeeds (i.e., FIG. 9,block 610, block 614, block 618). When the operation is complete, thefile system iterates over either the freed list or the allocated list(depending on the success of the operation) and adds these blocks to theglobal free list. Because commits are atomic, this data never needs tobe stored in NVM 110 or reconstructed.

Third, the file system stores a cache of directory entries from eachdirectory that has been opened by the user. Each directory entry in thecache is stored simultaneously in a list and a hash table so that thefile system can support quick, ordered directory listings as well asquick individual name lookups. Any updates to directories areimmediately reflected to the non-volatile memory component 110 as well.

Because these data structures are only found in the DRAM component 112,the file system need not use atomic writes to update them; rather, theyare synchronized with the file system updates using only conventionallocks. An alternate design might place some of these structures in thenon-volatile memory component 110 directly.

Non-Volatile Memory File System Operations

This section presents details of an embodiment of a file systemimplementation. Since files in the file system use the same basic treedata structure, the implementation has a core set of routines, calledthe crawler, which is designed to traverse these trees and perform readsand writes. To implement a file system operation, the crawler is given aroot pointer 602, the height of the tree, a range of file offsets, and acallback function. Because the system can compute the file offsetsspanned by each pointer, the crawler only needs to visit the pointersincluded in the specified range of offsets. Once the crawler gets to theleaf nodes, it will invoke the callback with the appropriate addresses.

The crawler is responsible for updating the tree height and any internalpointers. To update the tree height, the crawler looks to see if therequested file offsets are beyond the offsets spanned by the currentfile tree. If so, it increases the height of the tree by an appropriateamount. Each increase in the height of the tree is a simple operation:the crawler allocates a new pointer block, sets the first pointer inthis block to the old tree, and then sets the root pointer 602 to pointto this new block (along with the new height, encoded as low-orderbits). These updates can all be performed atomically, independent of thewrite operation that is about to be performed.

At leaf nodes, the crawler invokes a callback, and if the callbackwishes to perform a copy-on-write operation, it will allocate a newblock, perform any necessary updates, and return the pointer to that newblock. The crawler then updates any internal nodes (i.e., pointer blocks514) as appropriate. If no modifications are made by the callbacks, thecrawler returns the existing pointer block untouched. If only onepointer is modified by the callbacks, then the crawler commits thatoperation in-place. If more than one pointer is modified, the crawlermakes a complete copy of that pointer block, deferring the commit to ahigher level in the tree.

Sometimes only copy-on-write is allowed. For example, when a writeoperation proceeds down two branches of the tree, neither branch isallowed to commit in-place, since any commits need to happen at a commonancestor. This case also arises when the user performs a write that willupdate existing data and extend the end of the file. Because the filesystem needs to update both the file size and the root pointer 602atomically, the file system needs to perform a copy-on-write on theinode 516 itself, and the file system needs to disallow in-place commitsduring the file write.

Because the file system has two levels of tree data structures (i.e.,the inode file 502 and everything below it), many operations invoke thecrawler twice: once to find an inode 516 in the inode file 502, and asecond time to perform some operation on that inode 516. The callbackfor the top level crawl invokes the crawler a second time for thebottom-level file. Copy-on-writes can be propagated upward through bothinvocations of the crawler.

In an embodiment, the file system is implemented in the MicrosoftWindows® Operating System Driver Model, but the following presents asimplified view of these operations. When a file is opened, the filesystem operation parses the path and uses the directory entry cache tolook up the target file or directory. Because the directory entry cachestores complete directory information in the DRAM component 112, thisoperation only needs to access the non-volatile memory component 110 ifa directory is being opened for the first time.

If the file does not exist and a new file is created, the file systemclaims a new inumber from the free list and then writes a new inode to ablock 516 of the inode file 502 at the appropriate offset. Becauseinodes within blocks 516 are invalid unless referenced by a directoryentry, these updates can be performed in-place. Once the inode 516 isready, the file system writes a new directory entry into the parentdirectory. Once again, this update can be done in-place, because thedirectory entry is not valid until a nonzero inumber is written to theappropriate field. Finally, the file system updates the directory entrycache in the DRAM component 112.

It is noted that this entire operation can effectively be performed within-place updates to metadata; thus, file creation is consistent,synchronous, and extremely fast. A few extra writes may be required whenthe inode file 502 or directory file 518 is extended.

When a file is read, the file system invokes the crawler on theappropriate range of the file. The read callback copies data from thedata block 520 into a user-supplied buffer. No updates to file systemdata may be required, although the access time can be updated with anin-place atomic write.

When a file is written, the file system may perform a copy-on-write ofthe inode 516 itself, such that the operation uses a two-level crawl.The top level crawl operates on the inode file 502 and locates thetarget file's inode 516. Then the file system invokes the write crawleron the appropriate range of this file. The write callback determineswhether an in-place write is possible, and if so, the write callbackperforms that write. If not, the write callback makes a copy of theblock, updates the copy, and returns the copy to the crawler. Thecrawler then updates the internal nodes using the logic described above.

The file system atomically updates either the file size or the rootpointer 602 within the inode 516 as necessary. If both are updated, thena copy-on-write is performed on the inode block 516 itself, and the newversion is returned to the inode file crawler to be committed higher upin the tree. For efficiency, the file system updates the filemodification time separately. If atomicity is required, the file systemcan force a copy-on-write on every write operation.

When a directory is read, the file system loads the directory into thedirectory entry cache, if the directory is not already cached. The filesystem searches for the requested name, looks up all relevant inodes inthe inode file 502 from the non-volatile memory component 110, and fillsthe application's buffer. Loading a directory into the directory entrycache may also be performed with the crawler. The crawler is invokedwith the entire file range, so that the crawler receives callbacks forthe entire directory. At each data block 518, the crawler reads therelevant directory entries and enters them in the cache.

When a file or directory is closed the file system checks to see whetherthe file or directory has been marked for deletions by a separate callnot shown. If so, the file system deletes the file or directory bycrawling the directory file to the location of the directory entry andwriting a zero to the inumber field in-place. Because a zero inumberindicates an invalid directory entry, this atomic write instantlyinvalidates both the directory entry and the inode 516 to which itrefers. Finally, the file system updates the volatile data structures,including the free block list and the free inumber list.

This implementation exhibits many of the benefits of redesigning a filesystem for use on the non-volatile memory component 110 (FIG. 1).Through use of byte-level accesses, the file system can perform in-placeupdates for a large number of operations, and through use of an atomicwrite, the file system can provide strong consistency and safetyguarantees for arbitrarily large changes to the file system. On a systemthat requires block-based updates, the file system would not be able toachieve the same combination of high performance and strong guarantees.

In certain embodiments, file write times are not updated atomically withrespect to the write itself, because doing so would require all writeoperations to be propagated up to the inode itself using copy-on-write.Therefore, if a crash occurs between the write and the timestamp update,it is possible that the timestamp will not be up to date. This problemcould be addressed by implementing a wider atomic write primitive or bysqueezing the modification time and the root pointer into a single64-bit value.

Instant Boot

An application enabled by non-volatile, byte-addressable memory is“instant boot.” Operating systems may take several minutes to boot froma completely powered-down state. For example, certain operating systemsoffer “sleep” and “hibernate” features for partially shutting down acomputer. In sleep mode, an operating system can power down all devicesexcept for the DRAM component 112, and in hibernate mode, the contentsof the DRAM component 112 are copied to the hard disk 116 so that powercan be turned off completely. Hibernating saves more power, but alsotakes more time to enter and leave the hibernating state. Fundamentally,startup may be a problem that is I/O bound to persistent storage (i.e.,hard disk 116). A faster hard disk 116 results in faster boot time, butdata structures and file system objects may be to be copied across a busand into the DRAM component 112 to be made useful.

A simple form of instant boot may keep operating system data structuresin PRAM 110 instead of the DRAM component 112. In this case, sleep modewould not require any power. Therefore, such method provides the powersavings of hibernate mode with the speed of sleep mode. Existingoperating systems may include all the necessary code for managinghardware devices during sleep mode.

A second form of instant boot may use the non-volatile memory component110 as a backing store for operating system data state. The file systemcan take a snapshot of the DRAM component 112 after the computer (i.e.,computer device architecture 100) has booted and store this snapshot inthe non-volatile memory component 110. On the next boot, this snapshotcan be copied from the non-volatile memory component 110 to the DRAMcomponent 112 in order to skip past portions of the boot sequence.Components and devices should be placed into the correct state, andupdate performed on the saved memory image to account for changes intime, hardware configuration, and etc. It is noted that the specificoperating system data structures required to support this scenariodepends upon the operating system.

Instant boot benefits in two ways from a cache hierarchy design. First,the time to read OS data structures from the non-volatile memorycomponent 110 is reduced, because the non-volatile memory component 110might be faster than other storage mediums, and because the non-volatilememory component 110 is located closer to the CPU component 104. Second,since the non-volatile memory component 110 is byte addressable from theCPU component 104, OS data structures do not need to be marshaled beforethey are written, since they must be when they are written to othermediums such as disk. Pointers can be preserved in the non-volatilememory component 110, and then restored when they are read back in theDRAM component 112 at the time of boot.

Application Checkpoints

The non-volatile memory component 110 opens up a new set of uses toapplications, including checkpointing application state. Applicationscould reflect the state of the application (e.g., heap, stack,registers) directly to the non-volatile memory component 110.Applications could use these checkpoints for reliability and security.

Extending Storage Capacity with Traditional Storage Media

Although the non-volatile memory component 110 provides many advantagesover hard disks 116 or solid-state drives for storing file system data,in certain cases, such a use maybe more expensive per byte than theseother media. Therefore, an approach to this problem may be to storemetadata and small files in the non-volatile memory component 110 basedfile system. For larger files that do not fit within the non-volatilememory component 110 based file system, the file system can store asymbolic link to a file stored on a hard disk 116 or solid-state driveusing a traditional file system such as NTFS. The operating system canimplement one of many policies for moving files between the non-volatilememory component 110 based and disk-based file systems in order to speedup access to commonly-used files while providing the high capacity of ahard disk 116.

The non-volatile memory component 110 based file system may beperiodically copied to the hard disk 116 itself so that recovery of thefull file system can be performed if only the hard disk 116 is removedfrom the system.

CONCLUSION

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described. Rather,the specific features and acts are disclosed as exemplary forms ofimplementing the claims. For example, the systems described could beconfigured as wireless communication devices, computing devices, andother electronic devices.

1-20. (canceled)
 21. A processor comprising: a control component thatenforces ordering and atomicity of memory writes; a memory component toenforce timing restraints in addition to the ordering and atomicityenforced by the control component; a non-volatile memory that isaddressable by the control component; and a dynamic random access memory(DRAM), which shares an address space with the non-volatile memory. 22.The processor of claim 21, wherein the memory component comprises: ascheduler component that provides timing to the control component; and amemory controller transaction component that includes data to be flushedaccording to the timing provided by the scheduler component.
 23. Theprocessor of claim 21, wherein the control component enforces orderingof writes by assigning epoch identifiers (epoch IDs).
 24. The processorof claim 21, wherein a persistence bit is used by the memory componentto reference data that is to be written in non-volatile memory.
 25. Acircuit comprising: a first component to enforce ordering and atomicityof writes to a memory component, the memory component addressable by afirst component and including a non-volatile memory which shares anaddress space with a dynamic random access memory (DRAM); and a secondcomponent coupled to the first component to enforce timing restraints onthe writes in addition to the ordering and atomicity enforced by thefirst component.
 26. The circuit of claim 25, wherein the firstcomponent enforces ordering of writes by assigning epoch identifiers(epoch IDs).
 27. The circuit of claim 25, wherein the second componentenforces ordering of writes based in part on a persistence bit and adirty block counter.
 28. The circuit of claim 25, wherein the firstcomponent is a central processing unit (CPU) and the second component isa memory controller.
 29. The circuit of claim 25, wherein thenon-volatile memory is phase change memory (PCM).
 30. The circuit ofclaim 25, wherein the non-volatile memory is byte-addressable.
 31. Thecircuit of claim 25, wherein the DRAM is used for stack or heap toimprove wear lifetime of the non-volatile memory.
 32. A processorcomprising: a persistent memory addressable by a control component toenforce ordering of writes; and a dynamic random access memory (DRAM),which shares an address space with the persistent memory.
 33. Theprocessor of claim 32, further comprising a memory component to enforcetiming restraints in addition to the ordering enforced by the controlcomponent.
 34. The processor of claim 32, wherein the persistent memoryis byte-addressable.
 35. The processor of claim 32, wherein thepersistent memory is capable of using small and random writes to reducetraffic in a memory bus.
 36. The processor of claim 32, wherein thepersistent memory is phase change memory (PCM).
 37. The processor ofclaim 32, wherein the control component enforces ordering of writes byassigning an epoch identifier (epoch ID) to the writes.
 38. Theprocessor of claim 32, wherein the control component enforces orderingof writes based in part on a persistence bit and a dirty block counter.39. The processor of claim 32, wherein the control component is acentral processing unit (CPU).
 40. The processor of claim 32, furthercomprising a memory bus coupled to the control component, the persistentmemory and the DRAM, wherein the persistent memory is addressable by thecontrol component via the memory bus.